1. Field of the Invention
The present invention relates to a pattern layout method of a photomask for pattern transfer that is used in photolithographic steps of a semiconductor manufacturing process. More specifically, the present invention relates to a pattern layout method of a photomask for pattern transfer having patterns m a belt form for transferring images to a photosensitive film and auxiliary patterns provided for securing a deep depth of focus.
2. Description of the Background Art
In recent years, the integration of semiconductor devices has been significantly increased. Though a variety of steps exist in the manufacturing process for a semiconductor device, it is widely recognized that photolithographic steps, in particular, are the key process steps leading to greater integration of semiconductor devices.
A variety of steps such as steps for forming element isolation regions, steps of forming wires, steps of forming electrodes and steps of forming contacts can be cited as manufacturing steps of a semiconductor device to which photolithographic technology is applied. As described above, an increase in the resolution in the photolithographic steps is essential for greater integration of semiconductor devices.
The photolithographic steps are the steps of transferring a light blocking pattern of a photomask to a photosensitive film, which has been applied to a semiconductor substrate, by irradiating the photosensitive film with light via the photomask. FIG. 6 is a conceptual diagram of a stepper utilized when the photolithographic steps are carried out. Light emitted from light source 1 passes through a fly-eye lens 2 and is condensed by a condenser lens 3 so that a photomask 4 for pattern transfer is irradiated with the light. The light that has been transmitted through a light transmitting portion of photomask 4 for pattern transfer passes through a projection lens 5 so that a photosensitive film 7, which is the material to be treated, on a semiconductor substrate 6 is irradiated with the light and the pattern in photomask 4 is scaled down and transferred to photosensitive film 7. As for the utilized light, an excimer laser beam or the like, in addition to a g-line beam and an line beam, are appropriately used.
Super resolution technology is one from among technologies that have been attracting attention as technologies capable of increasing the resolution of patterns in photolithographic steps. This technology is a technology for resolving patterns not conventionally resolvable that applies a variety of new ideas to respective portions of the optical system.
There exists a technology wherein a pattern (hereinafter referred to as auxiliary pattern) that does not substantially transfer an image to the photosensitive film is placed in the vicinity of a pattern (hereinafter referred to as main pattern) that transfers an image to the photosensitive film-as one type of the above described super resolution technology. Here, this auxiliary pattern is a microscopic pattern no larger than the resolution limit.
This technology controls the phase and intensity of light through the placement of the auxiliary pattern in the vicinity of the main pattern and improves the resolution of patterning in the photolithographic steps by improving the light intensity distribution for the main pattern at the time of exposure. Thereby, it becomes possible to transfer the objective pattern to the photosensitive film formed on the semiconductor substrate with a greater precision and, therefore, it becomes possible to form a semiconductor device provided with a microscopic structure according to the original design. In addition, it becomes possible to make the depth of focus deeper in the photolithographic steps by providing the auxiliary pattern and, thereby, it becomes possible to expand the range of the focal point in order to allow the resolution of the pattern even in the case that a shift in focal point has arisen.
Conventionally, the design of the main pattern has been carried out using CAD (Computer Aided Design). The layout of the auxiliary pattern is automatically arranged according to predetermined rules with respect to this layout of the main pattern.
As shown in FIG. 7, a main pattern 41a extending in a band form, located independently of other main patterns, has, for example, auxiliary patterns 42a on both sides so that auxiliary patterns 42a extend in parallel at a predetermined distance away from main pattern 41a. 
On the other hand, main patterns, making up a plurality, placed approximately parallel to each other in an array have auxiliary patterns, as shown in FIGS. 8 and 9, placed according to the pitch of these main patterns. That is to say, in a fine pitch layout wherein main patterns 51a to 51e are arranged in close formation, as shown in FIG. 8, auxiliary patterns 52a and 52e are provided solely outside of main patterns 51a and 51e, which are located on each end, while no auxiliary patterns are provided between main patterns 51a to 51e. This is because patterns are formed with a sufficient precision even in the case that auxiliary patterns are not provided.
In addition, in a middle pitch layout, as shown in FIG. 9, one auxiliary pattern 62ab, 62bc is placed between each pair, respectively, of main patterns 61a to 61c, in addition to auxiliary patterns 62a and 62c placed outside of main patterns 61a and 61c located on each end.
As described above, these auxiliary patterns are automatically placed according to predetermined rules with respect to main patterns. Therefore, in some cases patterns in forms as shown in FIG. 10A and FIG. 10B are formed in a photomask.
FIG. 10A shows the condition wherein a corner formed from the sides that form an end portion of pattern 9 makes contact with corner formed from the sides that form an end portion of pattern 8. In the following, this point of contact A is referred to as a “microscopic point of contact.”
In addition, FIG. 10B shows the condition wherein a side that forms an end portion of pattern 9 makes contact with a side that forms an end portion of pattern 8 so that the sides are partially overlapped. In the following, this contacting portion B is referred to as a “microscopic step”.
In the present invention, partially overlapped indicates a condition wherein a side of one pattern is partially included in a side of another pattern even though the side of the one pattern is not completely included in the side of the other pattern so that the concept of partial overlapping includes the conditions of FIG. 10A and FIG. 10B.
As cases in which such a microscopic point of contact or a microscopic step is generated, cases such as are shown in FIG. 11 and FIG. 12, for example, are possible. FIG. 11 shows a case of a layout wherein one main pattern 111b, from among two main patterns 111a and 111b in band forms placed parallel to each other in a fine pitch layout, is terminated partway. In this case, an auxiliary pattern is not formed between main pattern 111a and main pattern 111b. However, in a portion wherein main pattern 111b is terminated partway (portion not extending in a parallel manner), an auxiliary pattern 112a is placed at a position at a distance d1 from main pattern 111a so that auxiliary pattern 112a extends parallel to main pattern 111a in an independent pitch layout. At this time, as shown in the figure, auxiliary pattern 112a and main pattern 111b make, in some cases, contact with each other so that a microscopic point of contact or a microscopic step is generated.
In addition, FIG. 12 shows a case of a layout wherein one main pattern 121b, from among the two main belt-shaped patterns 121a and 121b arranged in parallel in a middle pitch layout, is terminated partway. In this case, an auxiliary pattern 122ab is placed between main pattern 121a and main pattern 121b at a position at a distance d3 from the respective main patterns. This auxiliary pattern 122ab is also terminated partway according to main pattern 121b. In addition, in a portion wherein main pattern 121b is terminated partway (portion not extending in a parallel manner), an auxiliary pattern 122a is placed at a position at a distance d2 from main pattern 121a in an independent pitch layout. Accordingly, as shown in the figure, in some cases this auxiliary pattern 122a and auxiliary pattern 122ab that is terminated partway make contact with each other so that a microscopic point of contact or a microscopic step generates.
A photomask for pattern transfer having such a microscopic point of contact or microscopic step causes a problem at the time of inspection for mask defects. Inspection for mask defects is an inspection to confirm whether or not patterns having the form as designed are formed in a photomask. At this time, in the case that there is a microscopic point of contact or a microscopic step in the photomask, as described above, it becomes difficult to determine whether such a microscopic point of contact or microscopic step has been formed according to original intentions or has generated as a defect at the time of mask manufacture. Usually, such an, inspection for mask defects is carried out using a device that automatically, performs inspection for defects based on a predetermined algorithm and, in such a case, the above described problem becomes significant so that the step of inspection for mask defects becomes considerably complicated.
Therefore, according to a prior art, as shown in FIGS. 13 and 14, a portion of auxiliary patterns forming a microscopic point of contact or microscopic step is removed in advance at the design stage of the mask pattern and ease of inspection for mask defects is taken into consideration. Concretely, in the case that an end side of auxiliary pattern 112a partially overlaps the end side of main pattern 111b, as shown in FIG. 11, end portion 112a3 of auxiliary pattern 112a is removed in advance, as shown in FIG. 13. In addition, in the case that an end side of auxiliary pattern 122a partially overlaps an end side of auxiliary pattern 122ab, as shown in FIG. 12, end portion 122a3 of auxiliary pattern 122a and end portion 122ab3 of auxiliary pattern 122ab are removed in advance. Thereby, a microscopic point of contact or a microscopic step can be prevented from being the cause of problems at the time of the inspection for mask defects and, therefore, inspection for mask defects can be made easy to carry out.
However, in the case that a portion of an auxiliary pattern is removed as described above, the resolution of the photosensitive mask due to the main pattern in the vicinity where a portion has been removed becomes insufficient causing the problem wherein the desired form cannot be obtained. In addition, in the worst case, a problem has arisen wherein no pattern is formed in this portion.
In addition, another problem arises wherein a gate pattern makes contact with an active region in the surface of the semiconductor substrate through the placement of an auxiliary pattern in the photomask for pattern transfer in the step of exposure that is carried out in the process of gate pattern formation. This is a problem that arises in the case that auxiliary patterns are automatically positioned according to predetermined rules based on the layout of main patterns.
This problem will be described in detail with reference to FIGS. 15A and 15B. In the case that an end side of auxiliary pattern 132a completely overlaps an end side of main pattern 131b, as shown in FIG. 15A, when transfer of patterns is implemented without the carrying out of any treatments on this auxiliary pattern 132a, a protrusion 134 may be generated at an end portion of gate pattern 133b made of a conductive layer, as shown in FIG. 15B. This is because the end side of main pattern 131b makes contact with the end side of auxiliary pattern 132a in the photomask so that the end portion of this auxiliary pattern 132a transfers the image. The generation of this unexpected protrusion allows gate pattern 133b to reach to a position above active region 201 so as to, in some cases, significantly affect the device characteristics.